1. Field of the Invention
The invention relates generally to seal assemblies for sealing between a rotating and a static member. In one particular aspect, the invention relates to seals for rolling cone bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. In a more particular aspect, the invention relates to seals that are employed to seal and protect the bearing surfaces between a rolling cone cutter and the journal shaft on which it rotates.
2. Background of the Invention
An earth-boring drill bit is typically mounted on the lower end of a drill string. With weight applied to the drill string, the drill string is rotated such that the bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A typical earth-boring bit includes one or more rotatable cone cutters. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts (also called journal shafts or pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, each cone cutter is caused to rotate on its respective journal shaft as the cone contacts the bottom of the borehole. The borehole is formed as the action of the cone cutters removes chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit. Liquid drilling fluid is normally used for oil and gas well drilling, whereas compressed air is generally used as the drilling fluid in mining operations.
Seals are positioned in glands formed between the rolling cones and their journal shafts to prevent lubricant from escaping from around the bearing surfaces and to prevent the cutting-laden, abrasive drilling fluid from entering between the cone and the shaft and damaging the bearing surfaces. When cuttings and/or abrasives are conveyed into the seal gland, they tend to adhere to the gland and/or seal component surfaces, and may cause deformation, damage and/or slippage of the seal components. Moreover, the cuttings can accelerate abrasive wear of all seal components and of the bearing surfaces.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the drill bit wears out or fails as a borehole is being drilled, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section in order to replace the bit. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. It is therefore advantageous to maximize the service life of a drill bit. Accordingly, it is always desirable to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rate of penetration (ROP).
The durability of a bit and the length of time that a drill bit may be employed before it must be changed depend upon numerous factors. Importantly, the seals must function for substantial periods under extremely harsh downhole conditions. The type and effectiveness of the seals greatly impact bit life and thus, are critical to the success of a particular bit design.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are typically subjected to high drilling loads, high hydrostatic pressures, and high temperatures.
As previously mentioned, the bearing surfaces in typical bits are lubricated. Seals between the journal shaft and the rotating cone mounted to the journal shaft serve to retain lubricant within the bit and prevent cutting-laden and abrasive drilling fluid from passing to the bit bearings. The seal is typically in the form of a ring disposed between the journal shaft and the rotating cone mounted to the journal shaft. Further, the seal typically includes a dynamic seal surface placed in rotating contact against the journal shaft surface, and a static seal surface that engages the rotating cone. In other words, this conventional seal rotates along with the cone relative to the journal shaft. Although the bit will experience severe and changing loading, as well as a wide range of different temperature and pressure conditions, the dynamic and static seal surfaces must nevertheless remain sealingly engaged in order to prevent the lubricant from escaping and/or cuttings from entering the lubricated areas, and should perform these duties throughout the life of the bit's cutting structure.
A variety of seal types are known in the art. These include O-ring type seals made of rubber or other elastomeric material. The service life of bits equipped with such elastomeric seals is generally limited by the ability of the seal material to maintain sealing engagement at each dynamic and static seal surface while withstanding the different temperature and pressure conditions at each dynamic and static seal surface.
The conditions at the dynamic seal surface of the seal are often more harsh than the conditions at the static seal surface. This is typically the case because the relative motion at the dynamic seal surface often results in increased friction, increased temperatures, increased abrasion, and increased wear. Consequently, failure of most conventional seals occurs at the dynamic seal surface that dynamically engages the journal shaft. Failure of the seal at the journal shaft surface may undesirably provide a relatively direct path for entrance of cutting-laden drilling fluid into the inner workings of the rock bit.
Internal pressures within rock bits are caused by the elevated temperatures that occur within a bit during operation, as well as the elevated temperature of the downhole environment. In some deep hole drilling applications, internal rock bit temperatures can go as high as 300° F. and beyond. During any drilling operation the pressure of drilling mud external the rock bit may be higher than 10,000 psi (˜68 MPa). This pressure is typically equalized within a bit by the pressure compensation subassembly, so that the annular seal has substantially equivalent pressure acting on both the mud side (i.e., the side of the annular seal positioned adjacent the bit external environment) and the bearing side (i.e., the side of the annular seal positioned distal the external environment and adjacent the bit bearing) of the seal. This pressure equalization is important for purposes of maintaining proper seal positioning within the seal gland in the bit.
However, in some cases, an unchecked differential pressure may arise and exert an undesired pressure force on the seal in an axial direction within the seal gland within which it is seated. The direction that the seal is urged depends on whether the bit external or internal pressure is controlling, which will depend on the particular bit design, drilling application and operating conditions. In situations where the bit external pressure is controlling, the annular seal will be forced inwardly within the seal gland in an axial direction along the journal axis. In situations where the bit internal pressure is controlling, the annular seal will be forced axially outwardly within the seal gland towards the bit external environment. In either case, if a sufficient pressure differential exists between the internal and external pressures, a sidewall portion of the seal may be urged and extruded into a clearance or groove extending from the seal gland that is formed between the cone and journal shaft. The portions of the seal that extrude into the clearance or groove adjacent the seal gland may become pinched between the rotating cone and static journal shaft, potentially leading to “nibbling” and/or excessive wear or damage to the seal. Due to the relative movement and harsher conditions encountered at dynamic seals, portions of the seal proximal dynamic seal surfaces are especially susceptible to undesirable extrusion and nibbling.
The internal temperatures within the drill bit, especially the temperature of sealing surfaces that engage seals between the journal shaft and the rotating cone mounted to the journal shaft, can impact the performance, wear, and lifetime of such seals within the drill bit. For instance, temperatures in excess of 250° to 300° F. may result in undesirable breakdown of elastomeric seals. In addition, higher seal temperatures generally cause the seal to expand. The expansion of the seal may result in increased engagement, friction, and wear at the dynamic seal surface, which in turn further increase the seals temperatures. This viscous cycle may result in relatively quick temperature increases in the seal, rapid wear of the seal, and eventually failure of the seal. In addition, excessive temperatures may result in undesirable breakdown in lubricant flowing between the journal shaft and rolling cone mounted to the journal shaft.
In many conventional multi-seal arrangements, a primary seal and a secondary seal are disposed proximal one another between the journal shaft and the rotating count mounted to the journal shaft. Traditionally, both the primary seal and secondary seal are oriented with their dynamic seal surface engaging the journal shaft. As previously described, heat generation and temperatures at dynamic seal surfaces are generally greater than static seal surfaces. By positioning the dynamic seal surfaces of both the primary and secondary seals adjacent one another against the journal shaft, the temperature of the journal shaft proximal the dynamic seal surfaces may increase beyond the desirable operating temperature range. The increased temperature of the journal shaft may lead to premature breakdown and/or failure of the primary and/or secondary seal.
It is therefore desirable that a new, durable and long lasting seal assembly be devised, one including long life, relative insensitivity to high temperatures and pressures, and enhanced protection of dynamic seal surfaces, but one that is not as susceptible to damage caused by extrusion and nibbling.
Accordingly, to provide a drill bit with better performance and longer life, and thus to lower the drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide a seal that has the potential to provide longer life than conventional elastomeric seals. Preferably, such seals would provide a bit that will drill with acceptable ROP for longer periods so as to increase bit life and increase in footage drilled as compared to bits employing conventional seals.